The present invention relates to an optical recording medium such as an optical card and an optical information recording/reproducing system for use with the optical recording medium. More particularly, the present invention relates to an optical recording medium on which high-density recording is advantageously achieved and also to a compatible, optical information recording/reproducing system which is applicable to both such a high-density optical recording medium and a conventional (less-density) optical recording medium.
Optical information recording/reproducing systems using an optical recording medium such as an optical card are popularly known today, and, in order for each of the recording/reproducing system to perform information recording on the optical recording medium, a plurality of optically detectable guide tracks are repetitively formed on the recording medium at predetermined intervals with a data track formed between every two adjacent guide tracks. Thus, the recording/reproducing system writes or reads information while accurately positioning a reading or writing optical beam at a desired data track on the basis of optical detection of the respective locations of the guide tracks (this technique is called "tracking"). Typical prior examples of the information recording and tracking techniques for such optical information recording/reproducing systems are disclosed in, for example, Japanese Patent Laid-open Publication Nos. SHO 60-229244, HEI 4-330647, HEI 4-341935 and HEI 5-101407 and Japanese Patent Publication No. HEI 4-38067.
FIG. 4 shows an optical recording medium (optical card 1) and a read/write optical system 2 employed in a typical example of a conventionally known optical information recording/reproducing system. The prior art pertinent to the present invention will be described hereinafter with reference to this figure. The optical recording medium is an optical card 1 that is of course portable and removably attachable to the read/write optical system 2. The read/write optical system 2 is accommodated within a casing (not shown) of the recording/reproducing device. Upon insertion of the optical card 1 into a predetermined insertion slot (not shown) formed in the casing, the card 1 is automatically transferred in the X direction by means of a card transfer mechanism (not shown) until the card 1 reaches a predetermined position for read/write operation by the optical system 2.
The optical card 1 has a substrate formed of light-weight hard material such as plastics and a recording medium portion (data recording region) 3 provided on a predetermined surface area of the substrate. The recording medium section (data recording region) 3 has a plurality of guide tracks 4 that extend in the card transfer direction X and are spaced apart from each other at predetermined intervals. There is formed a data track 5 between every two adjacent guide tracks 4. The guide tracks 4 are formed to be optically detectable. Using photoprinting process, for example, each of the data tracks 5 is formed to provide a relatively high reflection factor, while each of the guide tracks 4 is formed to provide a relatively low reflection factor. On the data tracks 5, desired information can be recorded in the form of circular holes called "pits". As is well known, the pits are formed by irradiation of a properly-focused write laser beam, and readout of recorded data on each data track 5 is achieved by detecting that the reflection factor of the laser beam irradiated onto the data track 5 is decreased by the "pit" portions. The recording medium portion 3 is composed of a recording layer formed of predetermined material that is reactive to the write laser beam and a protecting layer that is provided over the recording layer in such a manner to allow transmission therethrough of the read/write laser beam onto the underlying recording layer.
The optical system 2 is integrated as an optical head unit and is movable in the Y direction (direction transverse to the tracks 4 and 5) by means of an optical head transfer mechanism (not shown). Further, for tracking purposes, the respective irradiated spots of the laser beams (laser beam spots) are finely adjustable. In general, the tracking operation is performed using a control method commonly known as a three-beam method, which is based on the arrangement that three laser beams spaced apart from each other are emitted from the optical system 2, with the middle one of the three laser beams being associated with the data track 5 as a read/write beam and the two laser beams on both sides of the middle beam being associated with the track guides 4 as tracking beams. More specifically, by measuring the magnitude of respective reflection of the two side laser beams, the irradiated spots of the laser beams are servo-controlled so that the tracking beams are accurately associated with the guide tracks 4 on both sides of the data track 5 in predetermined positional relation thereto and consequently the middle read/write beam spot is always accurately at a substantially middle position of the corresponding data track 5.
To further explain the optical system 2 with reference to FIG. 4, scattering light is emitted from a light source which is, in this example, in the form of a laser diode 1, and the scattering light is then collimated through a collimator lens 20 and further split into three beams via a diffraction grating 21. There is provided an angle adjusting mechanism (not shown) to allow the diffraction grating 21 to stably follow the optical card tracks. After the diffraction grating 21 is a beam splitter 22 for separating the beams into transmitted and reflected light components. After the beam splitter 22 is provided a reflector mirror 2 that serves to change the direction of the laser beams by 90.degree. so as to achieve a compact size of the optical head unit (optical system 2). An objective lens 24 is further provided for converging and irradiating the individual laser beams onto the optical card 1. There are provided a vertical drive mechanism 17 for focusing the objective lens 24 and a tracking drive mechanism 18 for moving the objective lens 24 transversely (in the Y direction) for tracking purposes. Thus, focusing and tracking operations are performed as desired by biaxially driving the objective lens 24 by means of these mechanisms 17 and 18.
The three laser beams irradiated through the objective lens 24 onto the optical card 1 are reflected with variable reflection factors depending on the presence or non-presence of the pit and guide track 4 and pass through the objective lens 24, reflector mirror 23 and beam splitter 22 where they are respectively separated as reflected light components. The respective reflected light components then lead to a light receiving system. The light receiving system generally comprises a collimator lens 27, a convex lens 26, a cylindrical lens 27 and a photo detector assembly 28, so that the respective reflected light components of the three laser beams are received by the photo detector 28 which in turn provides electrical signals corresponding to the energy levels of the light components received.
FIG. 5 shows an example of a tracking error detection circuit provided in the optical system. The photo detector 28 is composed of three photo detector elements 28a, 28b, 28c that are positioned at predetermined locations for detecting the corresponding reflected light components of the three laser beams: the side photo detectors 28a and 28b detect the reflected light of the two tracking laser beams, while the central photo detector 28c detects the reflected light component of the read/write laser beam. The tracking error detection circuit includes a differential amplifier 29, which receives at its "-" input one of the output signals of the tracking laser beam receiving photo detector elements 28a and 28b and receives at its "+" input the other of the output signals.
In FIG. 5, for ease of understanding, the two guide tracks 4 and intervening data track 5 and the three laser beam spots La, Lb, Lc are shown as overlapping the photo detectors 28a, 28b, 28c. The positional relationship between the laser beam spots La, Lb, Lc and the tracks 4, 5 is such that when, as shown, the middle read/write laser beam spot Lc is exactly in the middle of the data track 5, the inner edges of the guide tracks 4 on both sides of the data track 5 are positioned across the centers of the side tracking laser beam spots La and Lb, respectively. As may be readily understood, when the guide tracks 4 are displaced from the illustrated positions to the right relative to the laser beam spots La, Lb, Lc, the amount of the laser beam La applied onto the left guide track 4 increases, while the amount of the laser beam Lb applied onto the right guide track 4 decreases. In consequence, the light energy received by the left photo detector element 28a decreases, while the light energy received by the right photo detector element 28b increases. It should be apparent from the foregoing that when the guide tracks 4 are displaced from the illustrated positions to the left relative to the laser beams La, Lb, Lc, the opposite of the above-mentioned takes place.
If it is assumed that the tracking beam detection signals output from the photo detector elements 28a and 28b are Sa, Sb, the differential amplifier 29 calculates a difference "Sb-Sa". When, as shown in FIG. 5, the middle read/write laser beam Lc is located exactly in the middle of the data track 5 and the inner edges of the left and right guide tracks 4 are positioned across the centers of the left and right tracking laser beam spots La and Lb, respectively, the output signals Sa and Sb from the left and right photo detector elements 28a and 28b equal and thus the differential amplifier 29 provides a zero output. Once the guide tracks 4 are displaced from the illustrated positions to the right relative to the beam spots La and Lb, Sa&lt;Sb and thus the output signal from the differential amplifier 29 takes a positive value corresponding to the displacement. Conversely, once the guide tracks 4 are displaced from the illustrated positions to the left relative to the beam spots La and Lb, Sa&gt;Sb and thus the output signal from the differential amplifier 29 takes a negative value corresponding to the displacement. The positive or negative value output from the differential amplifier 29 is representative of a tracking error.
Accordingly, for tracking control, the tracking drive mechanism 18 is servo-controlled in the transverse direction (in the Y-direction of FIG. 4) in order to eliminate a tracking error, i.e., to make "0" the output from the differential amplifier 29. More specifically, if the output from the differential amplifier 29 is a positive value, the laser beam spots La, Lb, Lc are moved to the right by the amount corresponding to the positive value, and if the output from the differential amplifier 29 is a negative value, the laser beam spots La, Lb, Lc are moved to the left by the amount corresponding to the negative value.
With the optical information recording/reproducing system constructed in the above-mentioned manner, in order to increase the storage capacity of the optical recording medium without increasing the medium's physical size, i.e., in order to achieve high-density recording on the medium, it is sufficient to narrow the respective widths of and intervals between the tracks. In such a case, the degree to which the recording density can be increased depends upon the fabrication technique (track formation technique) employed for the optical recording medium, as well as upon the grating technique for forming the three laser beams separated at predetermined intervals.
However, the densification technique traditionally employed for optical recording media inevitably required not only narrowing of the widths and intervals of the tracks as mentioned above but also substantial structural alterations of the optical system so as to generate laser beams of suitable standard for the requirement. For these reasons, after the optical information recording/reproducing system was structurally standardized and various devices and components of the system were fixed to predetermined sizes, the recording medium to be used by the system could not easily be formed to high density. Namely, it is not sufficient to merely increase the recording density of the optical recording medium, because the optical system must itself be structurally altered. So, even if densification of the optical recording medium is at all possible from a technical point of view, there would be encountered the problem of high cost due to the required structural alterations of the optical system. In addition, because of the required structural alterations of the optical system, the prior art optical recording medium after being subjected to densification process becomes incompatible with the conventional systems and hence has reduced utility.
For example, to change the track intervals requires change in the intersecting angle .theta. of the row of the three laser beam spots with respect to the tracks; however, with the conventional optical systems, the grating must be adjusted to change the intersecting angle .theta. of the row of the three laser beam spots with respect to the tracks. In general, the grating adjustment is such an operation to bend, to a desired angle, a diffraction grating formed of rectangular thin flat glass when it is to be fixed to a housing made of plastics or sheet metal. This grating adjustment operation is performed relatively easily during manufacture of the device, but is very difficult to perform after the device is assembled within the optical head unit.